The present invention pertains generally to navigational devices and more particularly to VOR bearing indicators. Conventional VOR indicators use a round dial known as an omnibearing selector which is marked from zero to 360.degree., referenced against an index mark and normally rotated with a knob to the desired bearing. A meter on the device provides a left/right indication of the position of the aircraft to the radial bearing selected on the omnibearing selector. The pilot can therefore determine his location on a radial bearing from the VOR station by rotating the omnibearing selector until the meter is centered. Many times, however, it is desirable to have an automatic indication of bearing without the necessity of adjusting an omnibearing selector, especially in cases where time is of the essence, in a distress situation, for example. Additionally, where a pilot must triangulate his location, the omnibearing selector device is unable to provide a highly resolved bearing indication due to the nature of the mechanical devices used to indicate bearing. Additionally, these mechanical devices are naturally prone to failure after their fairly short lifespan is exceeded.
To overcome these disadvantages and limitations, various methods have been used to produce a device for electronically, digitally displaying VOR bearing. One method used in the prior art to produce an electronic signal indicative of VOR bearing has been to determine the difference in phase by counting the time difference between the positive going zero crossing of the reference signal and the positive going zero crossing of the variation signal for each cycle. In the presence of a noisy or fading signal, the zero crossings can vary to indicate bearings which are in error by 20.degree. or more. In addition, a digital readout does not display a stable signal on which the pilot can rely.
To overcome these problems, a method was developed whereby a plurality of count intervals based on zero crossings were calculated and then averaged in the electronic device. This method provided a stable readout in the presence of a noisy and fading signal. However, this averaging scheme presented problems when the aircraft was located near the zero degree radial from the VOR station. In this case, and especially when the aircraft is flying along or approximate to the zero degree radial, noise signals cause the reference and variation phase differences to occasionally jump from one side of the zero degree radial to the other. For example, if the pilot was flying along the 2.degree. radial in the presence of a weak, noisy and fading signal, atmospheric or multipath noise could cause the phase difference between the variation and reference signal to erroneously indicate to the equipment that the location of the aircraft was, for example, along the 358.degree. radial for at least one cycle. In an averaging scheme where small angles, such as 1.degree. or 2.degree., would be indicated by a very small count, large degree angles such as 358.degree. would be indicated by a large count. The presence of one or more erroneous pulses produced by noise during the averging cycle of either signal would cause an obviously erroneous average count. More specifically, if a series of one degree clock pulses is used to produce the counting pulses and the phase difference between the reference and variation signals is counted for 32 cycles, a total count pulse of 64 counts would be produced for a radial bearing of 2.degree.. This total of 64 would be averaged by dividing by 32 to indicate a bearing of 2.degree.. In the case where noise shifts a single pulse of either the reference or variation signals to indicate a phase difference between these signals of, for instance, 359.degree., a count of 358 would be averaged for one interval with the remaining 62 counts and divided by 32 to obtain an average indicative of a radial bearing of approximately 13.degree.. Even more error is obtained if more than one pulse is shifted by noise. The same is true for the case where aircraft is located at a very high degree radial, such as 358.degree., and noise shifts one or more pulses so that small counts are being averaged with larger counts to indicate an erroneous bearing.